Shannon Briggs, age 10, of Lansing, MI . , for her question:

How old are Carlsbad Caverns?

The great caverns are like a vast underground city carved into a massive bed of limestone. The stone was laid down underwater some 200 million years ago when this whole area was covered by a finger of the sea: Later, the Rocky Mountains began to poke up their proud heads and a huge ridge lifted high in the air. The upheaval lifted the Carlsbad area above sea level. As this happenec ‑ slowly, very slowly the lapping water bit into the soft limestone, washing away tunnels and caves, caverns and towering cathedrals. This tremendous job of stone work was done between 50 and 60 million years ago.